An electric contact switching device that mechanically makes and breaks an electric current, a relay and a sliding contact, etc. has features such as very low energizing resistance in making state, very high isolating electric resistance in off-state, excellent isolation between a control signal and a making/breaking contact circuit, and comparatively cheap manufacturing costs, compared with semiconductor switches. Therefore, it is widely used to make and break connection in an electric circuit where a power supply, an actuator, and a sensor, etc. are included in all fields such as information instruments, industrial equipments, cars, and consumer electronics. Moreover, it is said that the production of mechanical switches and the relays will keep increasing in the future.
A conventional electric contact switching device consists of a pair of electric contacts for making and breaking operations of an electric switching circuit. During breaking operation of a pair of electric contacts, the contacting area of each electric contact becomes narrow, and the current concentrates into only one contacting point, a molten metal bridge between contact electrodes glows and the bridge lastly evaporates. Further current concentration will lead to metal evaporation.
For the making and breaking operation of the large current from the high voltage power supply using the conventional electric contact switching device, whenever the energizing contact current exceeds the minimum arc discharge ignition current (minimum arc current) and the contact voltage exceeds the minimum arc discharge ignition voltage (minimum arc voltage), the arc discharge is inevitably ignited (for example, see Non-Patent Documents 1 to 4). The minimum arc current and the minimum arc voltage are decided depending on the kind of the electric contact material. The arc discharge in the contact is accompanied by heat generation at the electrodes and transfer of the contact material, and decreases the reliability and the lifetime of the relay for the large current making/breaking operation.
The conventional electric contact switching device consists of a couple of contact electrodes made of Cu metal plated with Au, Ag, Pd, or Pt for example, of which the resistivity is very low, and the contact resistance is very low. In order to suppress the arc ignition, the new materials with a high melting temperature and low resistivity for example, have been studied, and an atmosphere gas and the operation in vacuum have also been studied. However, there would be no applicable technology of arc ignition suppression for the conventional contact. To suppress the arc discharge as much as possible, heating the contact electrodes or decreasing the heat conductivity of the electrodes has been studied. However, it negatively affects a driving coil of the relay and its effect is so limited. The contact electrodes are sometimes mechanically divided into plurals to improve the reliability of contacts, and such contacts are called twin contacts. It has two mechanical springs for the making/breaking operation and for preventing an insulator obstacle of the contact, but not to suppress the arc ignition. The relay with two couples of contacts with different contact material has been proposed. They are operated in a timely-controlled manner. One contact, which operates earlier than the other, has low electric resistance for energizing the currents and the other contact, which operates behind the other, has high endurance to welding due to the arc ignition (see Patent Document 1). However, in this case, the arc ignition could not be suppressed. There would be no solution to suppress the arc ignition for the conventional contact.
In order to improve the reliability, high performance, miniaturization and low price, the following five problems are chiefly examined for the electric contact of the large current and the high voltage. The difficulty of problems mainly comes from the arc discharge during the breaking operation.
(1) Welding of the electric contacts
(2) Material transfer from the electrodes during breaking operations
(3) Contact resistance increase by chemical reaction or surface roughness (oxidation and sulfuration, etc.) on the surface of the electrode
(4) Miniaturization of the shape
(5) Decrease of serge generation
For the welding phenomenon of the electric contact in the above-mentioned (1), the molten metal bridge due to metal melting and metal evaporation generated by energizing current concentration into one spot is the main cause. It has a close correlation with the surface roughness and mass transfer of the electrodes due to the arc discharge. Because the arc current direction in a DC circuit is not changed, the problem for DC current switching is severer than for AC. The material transfer from the electrodes during the contact operations in the above-mentioned (2) is a complicated phenomenon of melting, evaporation, and the arc discharge. The contact resistance increase by the chemical reaction on the electrode surface in the above-mentioned (3) is induced by a rise in the metal temperature and an activated gas by the arc discharge. Miniaturization in the above-mentioned (4) of the contact device, the relay for example, is difficult due to the making/breaking mechanism against the arc discharge. The moving mechanism with a wide gap is inevitable to erase the arc discharge and a large contact force is necessary to overcome the roughened surface of the contact caused by the arc discharge for the low contact resistance. The serge generation in the above-mentioned (5) is induced inevitably at the breaking operation of the large current through an inductive load. When the large driving actuator breaks the large current with high velocity, bounce would occur and a complicated noise is generated due to a mechanical resonance of the moving electrode. Therefore, stable arc discharge, which starts as arc of the vaporized metal due to the arc discharge at the breaking operation and transfers to the arc of a surrounding gas, deteriorates the contact characteristics due to material consumption, material transfer and oxidation of the electrodes. If the arc ignition of the contact is suppressed, a lot of problems of the electric contact would be drastically solved.
Other than the electric contact switching device, generation of the arc discharge is also a problem for an armature of an electric motor or a pantograph of a train. For increasing the power consumption in the electrical equipments of cars, the higher voltage power supply is required for reducing electric power dissipation of a wiring. In the home electronics, a 300 V AC source would become popular for the higher power equipments. Therefore, the arc ignition of the electric contact becomes more important problem and the countermeasure has been studied eagerly.
As the arc discharge is estimated to be inevitable, metal composition, a thickness, a structure and a gap length of the contact electrodes are designed following to their deterioration factors to endure the target number of making/breaking cycles. Table 1 shows well known values of the minimum arc discharge current Im and the minimum arc discharge voltage Vm (see Non-Patent Document 5). For the electric contact of Au metal, the minimum arc discharge current Im is 0.38 A, and the minimum arc discharge voltage Vm is 15 V as shown in Table 1.
Table 1 shows the minimum arc electrical discharge currents and the minimum arc electrical discharge voltages in various metallic materials.
TABLE 1Determinations of Im and Vm in normal atmosphere, by various observers;Electrode diameter >> diameter of cathode spot; cf. Table (X, 3)ImVmAVMaterialIVESFINKHOLMIVESGAULRAPPFINKHOLMC0.020.0115.520Al18.314Fe0.730.35 to 0.558.013 to 15Ni0.20.58.014Cu1.150.4312.58.513Zn0.36(0.1)10.99.010.5Ag0.90.412.3812Cd(0.1)9.811Sb9.910.5Ta0.598W1.751.271.0 to 1.115.21015Pt0.671.00.7 to 1.11515.313.517.5Au0.380.420.3811.512.69.515Pb0.529.17.5
In order to quench the ignited arc discharge, a capacitor connected in parallel to the electric contact has been used as a quenching circuit. That is, the arc current at the contact is divided into the capacitor, the arc current becomes lower than the minimum arc discharge current, and the arc discharge disappears. For instance, it was reported that the minimum arc of Au is improved from 0.38 A to about 6 A by connecting the capacitor of 1 μF with the electric contact. However, there is a problem that the capacitor with the contacts decreases impedance for the AC current and is limited to the DC current. It means that isolation characteristics of the contacts decreases and the applicable circuits would be so limited. Adding to it, if the contacts are made during the capacitor is filled with high voltage charge, the rush current from the capacitor to the contact would raise the temperature of the metal contact and would cause welding of the contacts. To decrease the rush current from the capacitor to the contact, the resistance connected in series with the capacitor was proposed. However, the applicable circuits would be limited. A theoretical examination of the principle using the parallel capacitor to increase the minimum arc discharge current is insufficient, and the relation between the intercepted current and capacity of the capacitor and the high speed current change have not been theoretically examined.
A problem with the contact device other than the arc discharge is that the temperature near the contact surface rises by current concentration upon the breaking operation, leading to melting or evaporation of the metal. There is a theory, the “φ-Θ theory”, which can presume the highest temperature Tmax near the contact surface from the contact voltage VC of the electric contact (for example, see Non-Patent Document 5). Provided that ρλ=LT (the Wiedemann-Franz law) is formed, where an isothermal surface temperature on both ends of a current path is the room temperature (T0=300k) and the contact voltage is Vc, an approximate calculation of Formula (1) is obtained.Tmax=((Vc2/4L)+T02)1/2≦3200·Vc[K]  (1)Here, potential differences corresponding to a softening point temperature Ts, a melting point temperature Tm, and a boiling point temperature Tb of the current path material are called a softening voltage Vs, a melting voltage Vm, and a boil voltage Vb, respectively.
To overcome these problems, the arc quenching circuit comprising the capacitor connected in parallel to the electric contacts and another electric contact connected in series with the above-mentioned connectors with the capacitor was proposed. The two contacts synchronously perform the making or breaking operation (see Patent Document 2).
[Non-Patent Document 1] Tasuku Takagi, “Phenomenon of electrical discharge of arc of electric contact”, Corona Publishing Co., Ltd, 1995
[Non-Patent Document 2] Atsuo Takahashi, “Research on generation area of point of contact arc”, Nippon Institute of Technology research report, 1976, Separate volume 1, p. 65
[Non-Patent Document 3] “Relay technical booklet”, Fujitsu component, 2002, p. 337
[Non-Patent Document 4] A. Hamilton and R. W. Sillars, “SPARK QUENCHING AT RELAY CONTACTS INTERRUPTING DC CIRCUITS”, P.IEE, United States, 1949, Vol. 96, p. 64
[Patent Document 1] Japanese Unexamined Utility Model Publication (Kokai) No. 06-70143
[Non-Patent Document 5] R. Holm, “Electric Contact Theory and Applications ”, United States, Springer-Verlag, New York, 1967, 4th ed., p. 283, p. 60
[Patent Document 2] Japanese Unexamined Patent Publication (Kokai) No. 09-245586